Discharge circuit for pulsed laser and pulsed power source

Details Discharge circuit for pulsed laser and pulsed power source Discharge circuit for pulsed laser and pulsed power source

1998-10-05

2002-05-14

Davie, James W. (Department: 2881)

Coherent light generators

Particular component circuitry

For driving or controlling laser

C372S038070, C372S055000

Reexamination Certificate

active

06389049

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement to the discharge circuit for a pulsed laser, for effecting pulsed laser oscillation by pulsed discharge at a prescribed repetition cycle so as to excite a laser medium, wherein variations in laser output caused by overshoot current at the time of discharge are eliminated.
Also, the present invention relates to a pulsed power source apparatus in which a pulse generating circuit using a power semiconductor switch is combined with a magnetic pulse compressor circuit, for generating pulses of large current with narrow width at a rapid repetition rate, and more particularly, to a pulsed power source apparatus which eliminates unstable load operations caused by energy not consumed by a load when pulsed current is supplied to the load, and which ensures magnetic resetting of the pulse transformer.
2. Description of the Related Art
“TEA laser” refers to a laser generated by a method in which laser oscillation is effected by applying electrical discharge to a gas at a pressure of one atmosphere or higher. In the TEA laser, a uniform glow discharge is generated between a pair of mutually opposite main discharge electrodes to form an inverted population area necessary for laser oscillation. In order to attain a glow discharge spread throughout the entire discharge space, it is necessary to perform preionization before the start of the main discharge and to ionize the entire main discharge space in advance. For an excimer laser in particular, it is necessary to have as much ionization as possible directly before the main discharge, because of the short life of electrons in the ionized gas.
Currently, such preionization is achieved by various methods such as using X-rays, spark discharge, corona discharge and the like. OF these methods, a method using corona discharge has been widely used because it is relatively simple and does not greatly contaminate the laser gas.
FIG. 10
shows an equivalent circuit of a conventional capacitor-transfer type magnetic pulse compression discharge apparatus for effecting preionization using corona discharge.
In the discharge circuit in
FIG. 10
, a corona preionization electrode
4
is placed at a side of a main discharge space
3
formed between a pair of main electrodes
1
,
2
. Preionization of the laser medium in this main discharge space between the main electrodes
1
,
2
is brought about with UV light generated by corona discharge by the corona preionization electrode
4
.
In this configuration, a capacitor C
0
, connected to a high voltage power source HV, is charged with electrical charge from the power source HV.
Next, when a switching circuit SW, comprising a thyratron, GTO or the like, is turned ON, current I
00
flows into a magnetic switch SI
1
, comprising the capacitor C
0
, switching circuit SW, capacitor C
1
and saturable reactor. After that, when the voltage in the capacitor C
1
rises to a prescribed voltage, the magnetic switch SI
2
becomes saturated and enters a low impedance state.
As a result, current I
01
flows through a loop formed by the capacitor C
1
, main discharge capacitor Cp and magnetic switch SI
2
, and the voltage in the capacitors Cp, Cb rises.
Afterwards, the voltage at the corona preionization electrode
4
rises by means of the preionization capacitor Cb to a prescribed voltage at which preionization starts. At that time, corona discharge is generated in the corona preionization electrode, current I
02
flows through the main discharge space
3
, and preionization occurs in the main discharge space
3
.
Afterwards, the voltage in the main discharge capacitor Cp rises as charging takes place. When this voltage reaches a prescribed voltage at which the main discharge begins, the main discharge starts between the main electrodes
1
,
2
and current I
03
flows. Then, the laser medium is excited by the main discharge generated between main electrodes
1
,
2
and laser light is generated.
Then, the voltage of the capacitors Cp, Cb rapidly decreases as a result of the main discharge and, after a prescribed period of time, returns to the state before charging started.
Pulsed laser oscillation results from repeating such a discharge operation at a prescribed repetition cycle (pulse oscillation frequency) established in the switching circuit SW.
FIG. 11
shows the waveform of the voltage VD applied between the main electrodes
1
,
2
for the cycle of one pulse.
As discussed above, the voltage VD increases (in this drawing, becomes more negative) with the charging of the main capacitor Cp. When this voltage VD reaches the prescribed voltage at which the main discharge starts, the main discharge is generated. The voltage VD rapidly drops after the main discharge is generated and at this time, overshoot voltage, with a polarity opposite to that of the discharge voltage, is generated due to a transient phenomenon. Then, directly before the return to a steady state, a voltage Vd (shaded portion) is generated. This voltage Vd is thought to be a voltage reflected from the power source HV of the aforementioned overshoot voltage.
In other words, because the magnetic switches SI
1
, SI
2
are in a low impedance state after discharge, the overshoot current generated directly after discharge passes through magnetic switches SI
1
, SI
2
and flows to the power source HV, where the reflected voltage Vd is generated as a result of the reflected reverse current Id (dashed line in
FIG. 10
) flowing into the main discharge capacitor Cp. The main discharge becomes unstable due to this reflected voltage Vd and causes variations in laser output.
Further, a part Id′ of the reverse current Id flows into the preionization capacitor Cb. As a result, the preionization discharge becomes unstable, causing the preionization to become unstable and resulting in variations in laser output.
The phenomenon of the reflected voltage Vd is explained in more detail referring to
FIGS. 12-14
.
FIG. 12
shows an example of a conventional pulsed power source apparatus. In
FIG. 12
, a first-stage capacitor C
0
for power is provided in a pulse generating circuit
21
. This capacitor C
0
is initially charged by means of a high voltage charger
22
and, as a semiconductor switch SW is turned on, supplies pulsed current I
0
from the capacitor C
0
, via a reactor L
0
, to a pulse transformer PT.
A magnetic reset circuit MR
1
prevents magnetic saturation of the iron core of the pulse transformed PT by supplying DC bias current to the reset coil of the pulse transformer PT.
Two magnetic pulse compressor circuits
231
,
232
are connected in a cascade arrangement on the secondary side of the pulse transformer PT. In the first magnetic pulse compressor circuit
231
, the pulsed current I
1
whose voltage is raised by the pulse transformer PT, effects high voltage charging of the capacitor C
1
. This charged voltage in the capacitor C
1
actuates a saturable reactor SI
1
operating as a magnetic switch so that a narrow-width pulsed current I
2
, having undergone magnetic pulse compression, is supplied in the polarity shown in the drawing to the next magnetic pulse compressor circuit
232
. In the same way, as a result of the saturable reactor S
12
operating a magnetic switch, the magnetic pulse compression of the pulse width is carried out by the magnetic pulse compressor circuit
232
and the pulsed current I
3
is output with the polarity shown in the drawing.
Meanwhile, magnetic reset coils and magnetic reset circuits MR
2
and MR
3
are provided in saturable reactors SI
1
and SI
2
, respectively. These are excited and saturated with the reverse polarity by the supply of a direct current after saturation of the saturable reactors SI
1
, SI
2
.
The pulse output of the magnetic pulse compressor circuit
232
supplies narrow width, high voltage pulsed current to a load
24
such as a laser head chamber. In the load
24
, a peaking capacitor Cp is provided in parallel with the circuit of main discharge electrodes
1
,
2
and preionization electrode
4
.

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